1. Field
The present invention relates to a method of manufacturing a perpendicular magnetic recording medium substrate. In particular, the method is capable of reducing the waviness of all wavelength components. The invention also relates to a magnetic recording medium substrate manufactured by the method.
2. Description of the Related Art
As a technique for improving the recording density of magnetic recording media, a perpendicular magnetic recording method has been put into practical use instead of a longitudinal magnetic recording method according to the related art.
A hard disk device includes a magnetic recording medium (hereinafter, simply referred to as a medium) that can be rotated at a high speed and a magnetic head facing the medium. The magnetic head slightly floats and flies over the medium rotating at high speed, due to the air flow generated by the rotation, thereby writing signals onto the medium or reading signals written on the medium.
In general, the magnetic recording medium is formed by sequentially laminating (1) a non-magnetic metal underlayer, which is a Ni—P plated layer, (2) a Cr underlayer formed by, for example, a sputtering method, (3) a Co alloy magnetic layer, and (4) a protective layer, which is, for example, a carbon layer, on a disk-shaped substrate made of an aluminum-based alloy.
In the hard disk device, the recording density of signals onto the medium greatly depends on the distance (hereinafter, simply referred to as the amount of flying height of the head) between the medium and the magnetic head that flies over the medium. As flying height is reduced, the recording density increases. The surface of the medium needs to be as flat as possible for the magnetic head to stably glide over the medium with a small amount of flying height without contacting the surface of the medium. Since the magnetic layer and the protective layer on the medium are very thin, the surface of the underlayer needs to be flat in order to planarize the surface of the medium. In the manufacture of the magnetic recording medium, after the underlayer is formed, mirror-like finishing (hereinafter, referred to as a polishing or grinding process) is performed on the surface of the underlayer. Polishing is performed as follows: urethane pads are attached to the surfaces of an upper platen and a lower platen; a substrate having an underlayer plated thereon is interposed between the upper and lower platens; and slurry is supplied through a slurry supply hole provided in the upper platen while the upper platen and the lower platen rotate in a designated direction, thereby polishing the surface of the substrate. In general, for example, alumina or silica grains are used as the abrasive grains of the slurry and a polyurethane foam pad is used as a sliding/contact process material.
Japanese Patent Application Laid-Open (JP-A) No. 2007-250166 discloses a method of manufacturing a glass substrate for a magnetic disk which polishes the surface of the glass substrate using a polishing pad, such as a urethane foam pad, while supplying slurry to the glass substrate, thereby preventing the swelling and drooping of the edge of the glass substrate. The slurry includes an abrasive such as cerium oxide or colloidal silica. In the method, the concentration of the abrasive ranges from 25 wt % to 33 wt %, preferably from 27 wt % to 30 wt %, in order to prevent the swelling and drooping of the edge of the glass substrate.
In recent years, there has been a strong demand for increasing the recording density of the hard disk. To meet the demand, the amount of flying height of the head is reduced to, for example, about 5 nm. For the magnetic head to glide stably on the medium with a small flying height of about 5 nm, it is necessary to planarize the surface of the medium. In particular, a magnetic head having a magnetoresistive (“MR”) element has been generally used. However, the MR element is weak against heat and is likely to generate an erroneous electric signal due to heat caused by contact with the medium.
There is also the waviness of many wavelength components in the surface of the substrate. In particular, wavelength components of about 20 μm to 1000 μm greatly affect flying height stability for the stable flying of the magnetic head. Therefore, it is necessary to reduce the waviness of the wavelength components and improve the flying height stability of the magnetic head.